Common Node.js Singleton pattern

NodejsSingletonPattern.js

// Singleton.js
class Singleton {
  constructor(initValue = 0) {
    this.value = initValue;
  }
  setValue(value) {
    this.value += value;
  }
  getValue() {
    return this.value;
  }
}

Singleton.instance = null
Singleton.getInstance = () => {
  if (!Singleton.instance) {
    Singleton.instance = new Singleton();
  }
  return Singleton.instance;
}

module.exports = Singleton.getInstance();

// index.js
const Singleton = require('./Singleton');

const instA = Singleton;
const instB = Singleton;

instA.setValue(100);

console.log(instB.getValue()); // return 100